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CdTe Performance boosted with a pinch of selenium

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Researchers gain key insight into CdTe soaring efficiency

In collaboration with partners at Loughborough University in the United Kingdom, researchers at Colorado State University's National Science Foundation-supported Next Generation Photovoltaics Center have reported a breakthrough in how the performance of CdTe thin-film solar cells is improved by adding selenium. Their results were published in the journal Nature Energy earlier this month.

"Our paper goes right to the fundamental understanding of what happens when we alloy selenium to CdTe," said Kurt Barth, a director of the Next Generation Photovoltaics Center and an associate research professor in the department of mechanical engineering.

Until now, it was not well understood why the addition of selenium has clocked record-breaking CdTe solar cell efficiency - the ratio of energy output to light input - of just over 22 percent. Together with CSU collaborators W.S. Sampath and Amit Munshi, Barth and an international team have solved that mystery. Their experiments revealed that selenium overcomes the effects of atomic-scale defects in CdTe crystals, providing a new path for more widespread, less expensive solar-generated electricity.

The CdTe thin films that the CSU team makes in the lab use 100 times less material than conventional silicon solar panels. They are thus easier to manufacture, and they absorb sunlight at nearly the ideal wavelength. Electricity produced by CdTe photovoltaic cells is the lowest-cost available in the solar industry, undercutting fossil fuel-based sources in many regions of the world.

According to the paper, electrons generated when sunlight hits the selenium-treated solar panel are less likely to be trapped and lost at the material's defects, located at the boundaries between crystal grains as they are grown. This increases the amount of power extracted from each solar cell. Working with materials fabricated at CSU via advanced deposition methods, the team discovered this unexpected behavior by measuring how much light is emitted from selenium-containing panels.

As selenium is not evenly distributed across the panels, they compared luminescence emitted from areas where there was little-to-no selenium present and areas where the selenium was very concentrated.

"Good solar cell material that is defect-free is very efficient at emitting light, and so luminesces strongly," said Tom Fiducia, the paper's lead author and a PhD. student at the University of Loughborough, working with Michael Walls. "It is strikingly obvious when you see the data that selenium-rich regions luminesce much more brightly than the pure CdTe, and the effect is remarkably strong."

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